What is this pattern at the bottom of the default cube?
It is the 'active face', which is the face that was selected last. 

This can be useful for actions like transforming, where if you change the 'Pivot Point' to 'Active Element' you can scale towards or away from the 'active face'.

'Vertex' and 'Edge' selection modes also have 'active' elements and will display the last selected element lighter than the others. Object mode also does this when you have multiple objects selected, which can be used to (among other things) copy settings from the active object to the other selected objects.